In order to better understand the energy processes occurring during the impulsive phase of solar flares we compare observations with our magnetic model calculations. We study the 1N/M1.9 confined flare of 20 October 2003 observed during a Joint Observation Program (JOP157), and concentrate on the spectral analysis of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These X-ray observations are combined with those from the Solar and Heliospheric Observatory (SOHO) instruments, the French Italian magnetograph (THEMIS), and the Multi-channel Infrared Solar Spectrograph (MISS). The flare occurred in a complex active region, NOAA 10484, with a delta configuration. For model calculations we extrapolate magnetic field lines, which allows us to understand the magnetic topology of the region. This topology and the long impulsive phase of the flare with numerous peaks (GOES, RHESSI) suggest multiple magnetic field reconnection processes. The RHESSI images show a bright structure in hard X-rays (HXR) that could be the tops of the loops. We measure a significant displacement of this structure between the two main maxima of emission, which infers different sites for the reconnection process. The energy release processes can be understood by analyzing the RHESSI spectra using different models. First, using the thermal plus broken power law non-thermal component, we derive the low energy cutoff for the power law distribution of the high-energy electrons (≈25 keV). Then, we apply two models (thermal plus thick-target and thermal plus thin-target non-thermal component) that allow us to fit the observations. These two models lead to similar results; non-thermal energy contributes a significant amount (approximately 20%) of the total flare energy only during the first peak of the impulsive phase. This suggests that the energy that heats the chromosphere is transported mainly by thermal conduction. The temperature of the thermal plasma is 34 MK and 20 MK at the first and second peaks, respectively.